1. Field of the Invention
The field of the present invention relates to the field of semiconductor lasers and methods of combining the outputs of semiconductor lasers.
2. Background
In the current art, most high energy lasers are either chemical lasers or solid state lasers. Many high energy solid state lasers are optically pumped by semiconductor lasers. Even though the optical pump semiconductor diode lasers themselves may have wall plug efficiencies approaching 70-80%, the pumping quantum defect and the mismatch between the spectrum of the semiconductor diode laser and the gain medium absorption band of the solid state laser limit solid state laser efficiencies to around 20-25%. This low efficiency is common to all solid state lasers, whether they are of fiber or slab design. Furthermore, the waste heat which is a result of the low efficiency must be extracted and dissipated, adding to the solid state laser system mass and footprint. On the other hand, while chemical lasers have higher energy extraction efficiency, they are encumbered by heavy optical structures, feed systems and dangerous chemicals.
Semiconductor lasers work differently than solid state lasers. Because of this, laser specialists tend to consider them a separate category from solid state lasers, although strictly speaking semiconductors are solid state devices. Semiconductor lasers are based on semiconductor gain media where the optical gain is achieved by stimulated emission at an intraband transition under conditions of high carrier density in the conduction band. Most semiconductor lasers are fashioned as waveguides terminated with end mirrors and with electrical current flowing through the device perpendicular to the waveguide optical cavity axis. One notable exception is the vertical-cavity surface-emitting laser (VCSEL) that has its optical cavity axis along the direction of current flow rather than perpendicular to the current flow.
Semiconductor diode lasers are a pervasive technology with hundreds of millions of them sold annually across the globe. High efficiency and high power diode lasers have been built, in particular to optically pump high energy solid state lasers.
This invention is aimed at using such highly efficient and high power semiconductor lasers to produce high energy lasers (HEL) with output power exceeding one kilowatt. Since overall HEL system mass and footprint are inversely proportional to optical efficiency (i.e., higher efficiency means lower weight and smaller size), for lightweight and compact designs, it is highly desirable that the HEL systems composed of these high power laser diode gain media exhibit high overall optical efficiency, e.g., 70% or more.